The subject matter disclosed herein relates generally to apparatus and methods for semiconductor radiation detectors.
Radiation detectors may be used for ionizing radiation, such as Gamma Ray and X-Ray radiation. For example, radiation detectors made of Cadmium Telluride (CdTe) or Cadmium Zinc Telluride (CdZnTe or CZT) may be employed as detectors for medical imaging, including in nuclear medicine imaging applications. Radiation detectors may be employed in a wide variety of fields, for example, in connection with gamma cameras, single photon emission computed tomography (SPECT), positron emission tomography (PET), computed tomography (CT), space telescopes, or homeland security applications.
In direct conversion detectors, such as semiconductor radiation detectors, detected photons of ionizing radiation are absorbed in the detector bulk. Each of the absorbed photons creates, in the absorption location of the photon in the detector bulk, an electric charge consisting of multiple electron-hole pairs. A high voltage bias is applied across the detector bulk between the positively biased anodes and the negatively biased cathode. The negatively charged electrons are drifted toward the positively biased anodes and move toward the anodes as a group that may be referred to as an electron cloud. Similarly, but in the opposite direction, the positively charged holes are drifted toward the negatively biased cathode and move toward the cathode as a group that may be referred to as a hole cloud.
When the electron cloud (which has a total electrical charge expressed as (−q)) drifts toward the anode, a “mirror” electrical charge Qe is induced on the anodes. In the absence of small-pixel effect, Qe is proportional to the drift distance X of the electron cloud from the absorption location of the photon in the detector bulk to the anode. The electrical charge Qe may be given by:
                              Q          e                =                                            q              0                        ⁡                          (              t              )                                ·                      x            D                    ·                      e                                          -                                  t                  e                                            /                              τ                e                                                                        Eq        .                                  ⁢                  (          1          )                    
In Equation 1, q0 is the initial charge of the electron cloud produced by the absorption of the photon in the detector bulk at a distance X from the anodes, D is the detector thickness measured from the cathode to the anode, te is the drift time of the electron cloud from the absorption point of the photon (where the electrons-cloud is created) to the anodes, and τe is the lifetime of the electrons before they are recombined with holes or trapped by traps in a forbidden band-gap.
Similarly, for the same case described above, the hole cloud has a total electrical charge (+q) that drifts toward the cathode and induces a “mirror” electrical charge Qh on the anode. In the absence of small-pixel effect, Qh is proportional to the drift distance (D-X) of the hole cloud from the absorption location of the photon in the detector bulk to the cathode. The electrical charge Qh is given by:
                              Q          h                =                                            q              0                        ⁡                          (              t              )                                ·                                    (                              D                -                x                            )                        D                    ·                      e                                          -                                  t                  h                                            /                              τ                h                                                                        Eq        .                                  ⁢                  (          2          )                    
In Equation 2, th is the drift time of the hole cloud from the absorption point of the photon (where the hole cloud is created) to the cathode, and τh is the lifetime of the holes before they are recombined with electrons or trapped by traps in the forbidden band-gap.
The electrons and the holes are charged negatively and positively, respectively, and drift in opposite directions. Thus, the charges Qe and Qh that are induced on the anode have the same polarity. The charges Qe and Qh may be added to give a total induced electrical-charge QT that is given by:
                              Q          T                =                                            q              0                        ·                          x              D                        ·                          e                                                -                                      t                    e                                                  /                                  τ                  e                                                              +                                    q              0                        ·                                          (                                  D                  -                  x                                )                            D                        ·                          e                                                -                                      t                    h                                                  /                                  τ                  h                                                                                        Eq        .                                  ⁢                  (          3          )                    
In the case that the lifetimes of the electrons and the holes τe and τh are significantly longer than the drift times of electrons and the holes te and th to the anode and the cathode, respectively, then the electrons and the holes may reach the anode and the cathode without being recombined. In such a case, where τe and τh>>te and th, equation (3) may be expressed as:
                              Q          T                =                                                            q                0                            ·                              X                D                                      +                                          q                0                            ·                                                (                                      D                    -                    X                                    )                                D                                              =                      q            0                                              Eq        .                                  ⁢                  (                      3            ⁢            a                    )                    
In the case expressed by equation (3a), where QT=q0, if the electron cloud and the hole cloud are drifted all along their respective distances to the anode and the cathode, respectively, without charge-recombination loss, then there is a complete charge collection. In the event of complete charge collection, the total charge QT induced on the anode is fixed, does not depend on the absorption depth (location) X in the detector bulk, and is equal to the initial electrical-charge q0 of the electron cloud or the hole cloud.
However, with CdTe and CZT detectors, the drift time of the hole cloud to the cathode, due to low mobility, may be much longer than the lifetime of the holes, i.e. th>>τh. In such a case, as can be seen from equation (3), most of the holes in the hole cloud are recombined prior to arrival to the cathode, and thus do not contribute to the total charge QT induced on the anode.
In such a case, the high mobility of the electrons may result in a drift time of the electron cloud to the anodes that is much shorter than the lifetime of the electrons, i.e. te<<τe. As can be seen from equation (3), in such a case, most of the electrons in the electron cloud arrive to the anode and most of the electrons contribute to the total charge QT induced on the anode by a complete charge collection process.
In such a case, the electron cloud is almost the only contributor to the total charge QT induced on the anode, and. under the conditions th>>τh and te<<τe equation (3) may be expressed as:
                              Q          T                =                              q            0                    ·                      X            D                                              Eq        .                                  ⁢                  (          4          )                    
In such a case, the charge induced on the anode depends on the absorption location X of the photon in the detector bulk, also known as Depth-Of-Interaction (DOI). The dependency of the charge induced at the anode on the DOI causes the electrical signal at the anode to vary according to the DOI, X. The dependency of the charge on the DOI creates what may be referred to as low energy tail in the detector spectrum, which significantly reduces the detector efficiency as used for imaging when only a relative narrow energy window is used around the energy peak (events with complete charge collection) of the detector spectrum. The detector efficiency, as used herein, may be defined as the ratio between the number of photons detected in an energy window around the energy-peak of the detector-spectrum and the total number of photons detected in the detector-spectrum.
To overcome the problem of the anode signals in photon counting detectors being dependent of the depth-of-interaction (DOI) of the counted photons in the detector bulk, various solutions have been attempted in the past, including the small-pixels effect solution, and the very high voltage solution.
In the small-pixels effect solution, a configuration including small anodes configuration is used to produce what is known as the “small-pixel effect.” It may be noted that the small-pixel effect may become noticeable as the pixel size becomes less than half of detector thickness or smaller. Due to the small anodes, the induced charge on the anodes becomes significant only when the electron cloud moves in the close vicinity of the anodes. This makes the signals induced on the anodes independent of the DOI. The hole cloud that moves far away from the small anodes almost does not contribute to the signal from the anodes. Accordingly, in the existence of small-pixel effect, the signal at the anodes is independent on the DOI.
However, the small anodes may not produce an electrical field that is strong enough to ensure complete charge collection, or sufficient charge collection. Accordingly, a steering grid may be inserted between the small anodes, with the steering grid biased by a voltage that is lower than the voltage of the anodes. In such an arrangement, the steering grid assists the small anodes in creating an electrical field, in the detector bulk, that may be strong enough to ensure complete or near complete charge collection by the small anodes, and, at the same time, the lower voltage of the steering grid allows the electrons to be drifted toward the small anodes without being collected by the steering-grid.
While the small anodes that are combined with a steering-grid provide a configuration in which the signals from the anodes are generally independent of the DOI, such a configuration has a number of drawbacks. For example, such a configuration forces the use of a large amount of small pixels, which complicates the processing of the signals acquired from the large amount of pixels. As another example, the steering-grid requires a High Voltage (HV) bias, which is separated from the HV bias of the anodes. Such an arrangement complicates the fabrication and assembly of the detectors.
Another solution attempted in the past is the very high voltage solution. Under the very high voltage solution, the HV bias applied across the detector bulk between the cathode and the anodes is increased significantly, relative to normal operation conditions, to produce a relatively very high electrical-field in the detector bulk. Using such an approach, under a relatively very high electrical-field even a hole cloud that has generally low mobility may have sufficient drift time to allow the hole cloud to reach the cathode before being recombined and thereby, according to Eq. (3), reduce or eliminate the dependency of the signals of the anodes on the DOI. The very high voltage solution also suffers from a number of drawbacks. For example, increasing the HV increases the leakage current, which may result in degradation of the energy-resolution of the detector. As another example, the use of very high HV may cause voltage breakdown. Even minor voltage breakdown may produce false events in the detection of the detector.